1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a thermo-electric semiconductor device.
2. Description of the Related Art
Thermionic emission is a well known effect in which phonons add enough energy to charge carriers in a solid to allow the charge carriers to be ejected from the surface of the material. The charge carriers transport energy away from the bulk of the material in the form of an electrical current. The use of thermal energy to provide electrical current underlies the fundamental thermal to electrical conversion.
Quantum Mechanical tunneling (also known as direct tunneling) is also a well known effect. In direct tunneling, charge carriers can overcome a barrier by so called tunneling through the barrier. In a classical sense, a particle enclosed with a barrier cannot ever overcome the barrier, but in quantum mechanical systems, if the barrier is made thin enough, there is a certain probability that the charge carriers can overcome the barrier. This process is called tunneling in the literature. Tunneling starts to take effect if two surfaces are spaced apart by at most 10 nm and becomes very pronounced at distance scales of 1 nm.
Direct tunneling assisted thermionic devices use the tunneling effect to reduce the apparent work-function of the surface to allow thermionic emission to take place at greatly reduced temperatures. The work-function is a barrier to the electrons ejected from the surface of the material, with the tunneling reducing the effective barrier height. The electrons will be ejected from the surface of the material through the momentum distribution that they have within the material, this momentum distribution is due to the temperature of the material.
Tunneling assisted thermionic devices have been envisioned for some time. This work addresses techniques that allow these devices to be manufactured and operated at low cost. In order to realize the direct tunneling assistance of the thermionic effect, the device must comprise two parallel plates, which are held very close together (on the order of 1 nm) while maintaining very low thermal conductivity between the two plates. A thermal gradient is placed across the two plates, the heated plate will eject charge carriers which are collected by the low temperature plate and causes current to flow. The requirement for low thermal conductivity allows for a high thermal gradient to be placed across the gap between the plates.
Most common devices that have demonstrated tunneling assisted thermionic emission have used vacuum as the medium between the two plates as it has the most optimal thermal conductivity properties. The problem in the design of such a device is to ensure that the spacing between the plates is accurate and does not change with temperature.
To accurately control the spacing between the plates, an obvious solution is to provide a solid material that acts as an accurate spacer between the two plates. An ideal material will have very low thermal conductivity. However, even the materials that have the lowest thermal conductivity cannot be used, this is because the gap required (1 nm) is so small that the thermal resistance between the plates is too low to support an appropriate thermal difference. A calculation here is illuminating:
Material: SiO2, Gth=1 W/m·K, Thickness of material 1 nm, area of device perpendicular to heat flow 10 mm×10 mm=1×10−4 m2.
1×(1×10−4)/(1×10−9)=1×105 (W/K), in other words, this device could support up to 100 KW of heat and only have 1K (or 1 C) temperature drop across it. It is a very good thermal conductor even though the material itself has a very low thermal conductance.
Another well-known technique is disclosed on a thesis entitled “Thermionic-tunneling multilayer nanostructures for power generation”, published on 10 Apr. 2006, by Taofang Zeng. This thesis discloses a structure where dielectric nano-wires or nano-particles are sandwiched between two electrodes. However, in this technique, there is a problem that dielectric nano-wires or nano-particles are fixed unstably on the surfaces of the electrodes, and the dielectric nano-wires or nano-particles are arranged irregularly.